A glossary of solution terms

Two bottles can both say 0.1 and still be different solutions. One figure may be moles per litre, another grams per 100 grams, and a third a normality that only makes sense for one titration. This comparison is for the person who has to weigh a salt, dilute an acid, or write a specification that another bench can repeat. The decision is which pair of terms the recipe actually used, and which pair it confused.

Water grade still sits underneath every one of those numbers. How resistivity, organic carbon, and endotoxin part company is covered in laboratory water types and where they fail. Turning a chosen concentration into a measured pH is preparing a buffer and checking pH. Solids and concentrates are specified from the reagents and chemicals catalogue. The vessel that makes a litre honest is in beakers and flasks. Put the unit, not a nickname, on the quotation request.

Molarity against molality

Molarity is moles of solute in one litre of solution. You dissolve, you adjust, and you bring the meniscus to the mark. The number moves when temperature moves, because the solution expands or contracts and the litre is no longer the litre you prepared. A buffer made up at a cool morning bench and used from a warm afternoon bench is a slightly different molarity even when nobody added a drop.

Molality is moles of solute in one kilogram of solvent. The kilogram does not care that the laboratory warmed up, which is why physical-chemistry tables often use it. Biological recipes are usually written for volumetric flasks instead, so molality is the less convenient preparation even when it is the more stable unit.

In dilute aqueous work the two numbers sit close together, and people treat them as aliases. They are not aliases once the solution is concentrated, once the solvent is not water, or once density is required to turn a mass of solution into a volume. A concentrated acid stock is the usual place the alias breaks. If the recipe says molal and you make it molar, or the reverse, say so in the notebook.

Weight per volume against weight per weight

Percent weight per volume is grams of solute in 100 millilitres of solution. Percent weight per weight is grams of solute in 100 grams of solution. Percent volume per volume is a third unit, millilitres per 100 millilitres, and it is not either of those. None of them is a molarity.

The practical clash is a bottle of concentrated acid or a syrup-like stock whose label percent is weight per weight. Density is not 1 gram per millilitre, so 37 grams in 100 grams is not 37 grams in 100 millilitres. The molarity you need for a dilution comes from that percent and the density of the solution you opened, taken from the label or a table that matches that reagent. Inventing the density, or ignoring it, scales every later buffer.

A second clash is grammatical. Some benches weigh 10 grams, dissolve, and call the result 10 percent weight per volume whether the final volume is 100 millilitres or merely "about a hundred". The definition wants the solute inside 100 millilitres of finished solution. Weighing into 100 millilitres of solvent is a different, slightly more dilute solution. Write which one you did.

Normality against molarity

Molarity counts molecules, or formula units, in a litre. Normality counts equivalents in a litre. An equivalent is defined by the reaction in front of you: protons transferred in an acid-base titration, or electrons transferred in a redox titration. Change the reaction and the normality can change while the bottle sits still.

Sulfuric acid is the textbook warning. If the titration uses both protons, a 1 molar solution is 2 normal. If a method uses one proton, the same bottle is 1 normal for that method. A normality copied from an old teaching sheet into an enzyme buffer imports that silent factor. Keep normality only where a titration still speaks in equivalents, and write the reaction beside the number. Prefer molarity unless the method sheet still uses equivalents.

Buffer capacity against the useful pH window

The useful pH window is the range where both forms of the conjugate pair are present in a useful ratio, about one pH unit either side of the pKa. Beyond it, one form is scarce and the solution stops behaving like a buffer. The window follows the pKa at the temperature you care about. Good and colleagues mapped zwitterionic buffers into those windows. The paper is a list of candidates, not a licence to skip the meter.

Buffer capacity is how much strong acid or strong base the solution can accept for a given pH change. It is largest near the pKa, and it scales with the total concentration of the buffer pair. A 100 millimolar buffer near its pKa swallows far more acid than a 10 millimolar buffer at the same pH. A concentrated buffer used two units away from its pKa can still have some capacity and still be a poor design, because you are spending salt and ionic strength to prop up a pair that is mostly in one form.

The confusion is to treat the window as a capacity claim. A sentence such as this buffer covers pH 6 to 8 does not say whether your reaction's acid load will move the pH by a tenth of a unit or by a whole unit. Specify identity, pH, temperature, and concentration. If the assay produces or consumes protons, raise the concentration or move closer to the pKa. If it only has to sit still against carbon dioxide, a modest concentration inside the window may be enough, and extra buffer may add ionic strength you did not want.

Osmolality against osmolarity

Osmolarity is a paper sum of solute particles per litre, usually molar concentrations multiplied by how many particles a salt is supposed to release. It ignores incomplete dissociation and it inherits molarity's temperature dependence. Osmolality is osmoles per kilogram of solvent. A freezing-point or vapour-pressure osmometer measures something close to it. Cells respond to water activity, so culture media are specified as osmolality. The calculated sum and the measurement part company in concentrated solutions and in anything with a high protein or sugar load.

You can compute an osmolarity from a recipe in a few minutes and still not know the osmolality until you measure it, or until the manufacturer states a measured range under named conditions. Match the measurement, not the sum. When cells look stressed, measure, then change one component the method allows. Do not rename a spreadsheet osmolarity as osmolality on the label.

Activity against concentration

Concentration is how much of a substance you put in. Activity is the effective amount the equilibrium actually feels, concentration multiplied by an activity coefficient that depends on ionic strength, temperature, and the other ions present. In very dilute solution the coefficient sits near one and the distinction is academic. In a buffer with a physiological salt load it is not.

pH is an activity scale for hydrogen ions. A reading of 7 is not a measured concentration of 10 to the minus 7 moles per litre. NIST's calibration guidance treats the standard buffers as a metrology procedure, not as a conversion from a formula. The Henderson–Hasselbalch ratio is a concentration sketch. The meter answers an activity question. They drift apart when ionic strength, temperature, or junction potentials move.

Conductivity can catch a gross salt error. It does not report pH, osmolality, or a single activity coefficient. A resistivity number on the water is the same kind of ionic statement, as the water pillar explains, not a statement of every contaminant.

Where a swapped unit shows up as a failed result

A titration that needs twice the expected volume has often met normality, a missed density, or a hydrate. Recalculate from the label in the unit the method named before you restandardise the sample. A medium that still shrinks or swells cells after a paper match is an osmolality or salt-form miss: measure, do not rename the spreadsheet. A pH that misses the Henderson–Hasselbalch sketch is often activity, temperature, or a tired electrode. If the slope fails, you do not have a pH yet.

Heat, humidity, and the unit you thought was stable

A volumetric flask used well above the temperature engraved on the glass is not delivering the litre molarity assumes. Name that temperature when the assay is fussy. Humidity adds water to a hygroscopic solid while you weigh it, so the moles are low. Weigh promptly and record the hydrate. After a power cut, recalibrate the balance or the osmometer before the next comparison.

Safety and the limit of this page

Concentrated solutions used to build these numbers injure skin and eyes. Dilute them as the safety data sheet requires, adding acid to water. This comparison does not approve a procedure. Biosafety rules attach to any solution that has already held an organism. Renaming the concentration does not disinfect it.

What to put on the enquiry

State the chemical, the salt form and hydrate, the concentration with its unit, the temperature if the unit is per litre, the pH and the temperature of that pH if you need a solution rather than a solid, and the water grade from the water pillar if the assay cares. Ask whether a liquid concentrate is labelled weight per weight or weight per volume, and ask for density if you must convert. Ask for measured osmolality only if you will check it. A quotation request written that way can be compared with the certificate. A request that says "0.1 buffer" cannot.